1. Field of the Invention
The present invention relates to a semiconductor memory and method of driving the same and, more particularly, to a semiconductor memory and method of driving the same suitable for use in data rewrite.
2. Description of the Related Art
A flash memory is a nonvolatile memory which holds data by storing electrons into a nitride film formed between gate oxide films. This flash memory has the advantages that, e.g., the fabrication cost can be reduced because the cell structure is simple.
Conventional data write and erase operations in a NOR flash memory cell which holds data by storing electrons into a nitride film will be briefly explained below with reference to a sectional view of FIG. 9 which shows an example of the structure of a NOR flash memory cell.
Data write is performed by generating channel hot electrons. More specifically, about 9 V, about 5 V, and 0 V, for example, are applied to a control gate 24, to a drain 25, and to a source 26 and a substrate 27, respectively, thereby trapping hot electrons generated in a channel 28 near the drain 25 into a nitride film 29.
Data erase is performed by hot hole injection. More specifically, about −6 V, about 6 V, and 0 V, for example, are applied to the control gate 24, to the drain 25, and to the substrate 27, respectively, and the source 26 is floated. In this manner, holes generated by an inter-band tunnel current flowing from the drain 25 to the substrate 27 are injected into the nitride 29, thereby neutralizing electrons 30 trapped in this nitride film 29.
Data in a NOR flash memory cell which holds data by storing electrons into a nitride film is conventionally rewritten as described above.
When data rewrite is repeatedly performed by the above-mentioned method, however, as shown in FIG. 9, electrons 32 are trapped in an oxide film 31 formed below the nitride film 29. This may deteriorate the transistor characteristics, particularly, the subthreshold characteristic (repetitive data rewrite described above will be referred to as cycling hereinafter).
Generally, when holes are injected into an oxide film the trap level of the oxide film increases. Accordingly, when data is erased by hot hole injection described above, a large number of electrons 32 are trapped in the oxide film 31 formed below the nitride film 29. These trapped electrons 32 deteriorate the subthreshold characteristic.
When the subthreshold characteristic thus deteriorates, the relationship (Vg-Id curve) between a gate voltage Vg and a drain current Id changes as shown in FIG. 10 before and after cycling. As depicted in FIG. 10, compared to a Vg-Id curve 34 before cycling, a Vg-Id curve 33 after cycling indicates that a drain current Id at a high gate voltage Vg reduces. Consequently, the memory cell passes program verify even if the electron injection amount in the nitride film 29 is small.
That is, in this conventional semiconductor memory having the NOR flash memory cell which holds data by storing electrons into a nitride film, it is erroneously determined that predetermined electrons are stored in the nitride film 29, although the electrons 30 trapped in this nitride film 29 are few, because the electrons 32 are trapped in the oxide film 31. Accordingly, no satisfactory data holding characteristic of the memory can be obtained.
Additionally, the electrons 32 trapped in the oxide film 31 are present in an unstable region. Therefore, if exposed to a weak electric field or left to stand these electrons 32 readily escape to the substrate 27. Especially when left to stand at a high temperature, these electrons 32 escape to the substrate 27 more easily (leaving electrons to stand at a high temperature will be referred to as baking hereinafter).
When the electrons 32 trapped in the oxide film 31 escape to the substrate 27 by baking or the like, the Vg-Id curve 33 after cycling shown in FIG. 10 changes to a Vg-Id curve 35 after baking shown in FIG. 10.
That is, a flash memory cell after baking restores its subthreshold characteristic compared to the cell after cycling, but increases a drain current Id at a high gate voltage Vg compared to the cell before cycling. Since electrons stored in the nitride film 29 are few in this case, the memory cell cannot pass program verify.
This decreases the read margin of, e.g., “data 0” (written state), so readout data changes. This may deteriorate the data holding characteristic of the memory.